The present invention relates to metal powder compositions. Particularly the invention relates to iron-based compositions suitable for compaction at elevated temperatures.
The powder metallurgy art generally uses different standard temperature regimes for the compaction of a metal powder to form a metal component. These include chill-pressing (pressing below ambient temperatures), cold-pressing (pressing at ambient temperatures), hot-pressing (pressing at temperatures above those at which the metal powder is capable of retaining work-hardening), and warm-pressing (pressing at temperatures between cold-pressing and hot-pressing).
Distinct advantages arise by pressing at temperatures above ambient temperature. The tensile strength and work hardening rate of most metals is reduced with increasing temperatures, and improved density and strength can be attained at lower compaction pressures. The extremely elevated temperatures of hot-pressing, however, introduce processing problems and accelerate wear of the dies. Therefore, current efforts are being directed towards the development of metal compositions suitable for warm-pressing processes.
The U.S. Pat. No. 4,955,798 (Musella) describes warm compaction in general. According to this patent, lubricants generally used for cold compaction, e.g. zinc stearate, can be used for warm compaction as well. In practice, however, it has proved impossible to use zinc stearate or ethylene bisstearamide (commercially available as ACRAWAX(copyright).), which at present are the lubricants most frequently used for cold compaction, for warm compaction. The problems, which arise, are due to difficulties in filling the die in a satisfactory manner.
The U.S. Pat. No. 5,744,433 (Storstrom et al) and U.S. Pat. No. 5,154,881 (Rutz) disclose metal powder compositions including amide lubricants, which are especially developed for warm compaction.
The lubricant according to the U.S. Pat. No. 5,744,433 contains an oligomer of amide type, which has a weight-average molecular weight Mw of 30,000 at the most. Very high densities and green strengths may be obtained by warm compacting powder compositions when the lubricant has a molecular weight above 4000, the preferred lubricant molecule having a molecular weight of about 6500. It has however been found that this lubricant has a tendency of sticking to the die wall, which requires frequent cleaning of the die. Another disadvantage is that the obtained green bodies are stained.
In the U.S. Pat. No. 5,154,881 the amide lubricant consists of the reaction product of a monocarboxylic acid, a dicarboxylic acid and a diamine. The only lubricant tested according to this patent is ADVAWAX(copyright) 450, the composition of which is not described in detail but the reaction product obtained includes i.a. ethylene bisstearamide according to Chemis-CIVS. Our experience of this product is that it is difficult to obtain a constant composition and quality, which in turn may result in components of varying quality. This may cause problems when the lubricant is used in large scale industrial production.
An object of the present invention is to reduce or eliminate current problems associated with large scale production.
A second object is to provide a new type of lubricant useful in metal compositions intended for compaction at elevated temperatures.
A third object is to provide a metal powder for producing components without stains.
A fourth object is to provide a metal composition including lubricant, which during the compaction of the metal powder does not deposit on the die wall.
These objects are achieved by using a powder composition comprising an iron-based powder and new oligomer amide type lubricant. The composition may also include one or more additives, such as binders, flow agents, processing aids and hard phases.
The warm compaction may be performed by mixing an iron-based powder with the oligomer amide type lubricant and optionally a binder, preheating the powder composition and compacting the metal-powder composition in a pre-heated tool.
The new amide type lubricant used according to the present invention may be represented by the following formula
Dxe2x80x94Cmaxe2x80x94Bxe2x80x94Axe2x80x94Bxe2x80x94Cmbxe2x80x94D
wherein
D is xe2x80x94H, COR, CNHR, wherein R is a straight or branched aliphatic or aromatic group including 2-21 C atoms
C is the group xe2x80x94NH (CH)n COxe2x80x94
B is amino or carbonyl
A is alkylen having 4-16 C atoms optionally including up to 4 O atoms
ma is an integer 1-10
mb is an integer 1-10
n is an integer 5-11.
It is preferred that D is COR, wherein R is an aliphatic group 16-20 C atoms, C is xe2x80x94NH (CH)n COxe2x80x94 wherein n is 5 or 11; B is amino; A is alkylen having 6-14 C atoms optionally including up to 3 O atoms, and ma and mb which may be the same or different, is an integer 2-5.
Examples of preferred lubricants to be used in the iron based compositions according to the present invention are:
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]2xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]2xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]2xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]3xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]3xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]3xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]3xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]4xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]4xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]4xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]4xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]5xe2x80x94OC(CH2)16CH3 
CH3(CH2)16COxe2x80x94[HN(CH2)11CO]5xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]5xe2x80x94OC(CH2)16CH3 
Other examples are
CH3)COxe2x80x94HN(CH2)5COxe2x80x94HN(CH2)2NHxe2x80x94OC(CH2)5NHxe2x80x94OC(CH3) having the MW 370.49;
CH3(CH2)2OCOxe2x80x94HN(CH2)11COxe2x80x94HN(CH2)12NHxe2x80x94OC(CH2)11NHxe2x80x94OC(CH2)20CH3 
xe2x80x83having the MW 1240.10
CH3(CH2)20COxe2x80x94[HN(CH2)11CO]10xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]10xe2x80x94OC(CH2)20CH3 having the MW 8738.04
CH3(CH2)4COxe2x80x94[HN(CH2)11CO]3xe2x80x94HN(CH2)12NHxe2x80x94[OC(CH2)11NH]3xe2x80x94OC(CH2)4CH3 
xe2x80x83having the MW 1580.53
CH3(CH2)4COxe2x80x94[HN(CH2)5CO]7xe2x80x94HN(CH2)6NHxe2x80x94[OC(CH2)5NH]7xe2x80x94OC(CH2)4CH3 
xe2x80x83having the MW 1980.86
CH3(CH2)20COxe2x80x94[HN(CH2)5CO]7xe2x80x94HN(CH2)6NHxe2x80x94[OC(CH2)5NH]7xe2x80x94OC(CH2)20CH3 
xe2x80x83having the MW 2429.69
and
CH3(CH2)16NHxe2x80x94[OC(CH2)11NH]4xe2x80x94CO(CH2)10COxe2x80x94[HN(CH2)11CO]4xe2x80x94HN(CH2)16CH3 
xe2x80x83having the MW 2283.73
The chemical differences between the new lubricant and the lubricant described in the U.S. Pat. No. 5,744,433 are that the new molecule has a central diamine or diacid moiety and identical terminal groups on both ends. The chemical difference between the new lubricant and the lubricant described in the U.S. Pat. No. 5,154,881 is that the new lubricant molecule includes the unit xe2x80x94NH(CH)nCOxe2x80x94. In contrast to the lubricant known from U.S. Pat. No. 5,154,881 no EBS is formed when the lubricant according to the present invention is prepared. EBS has the chemical formula CH3(CH2)16COxe2x80x94HN(CH2)2NHxe2x80x94OC(CH2)16CH3) is a molecule without lactam units which is in contrast to the lubricants according to the present invention.
As regards the molecular weight of the new lubricant molecule it has been found that the preferred lubricants have a molecular weight between 1000 and 5000, most preferably between 1500 and 3000.
The lubricant molecule may be prepared according standard procedures for amide oligomer as described in e.g. xe2x80x9cPrinciples of Polymerizationxe2x80x9d third edition by George Odian (John Wiley and Sons, Inc.). According to the present invention the lubricant preferably consists of at least 80% of the amide having the formula described above. Thus up to 20% by weight of other types of lubricants may be added, as long as the advantageous properties of the new lubricant is not detrimentally affected.
This lubricant, which is added to the iron-based powder is preferably in the form of a solid powder, can make up 0.1-1% by weight of the metal-powder composition, preferably 0.2-0.8% by weight, based on the total amount of the metal-powder composition. The possibility of using the lubricant according to the present invention in low amounts is an especially advantageous feature of the invention, since it enables high densities to be achieved.
As used in the description and the appended claims, the expression xe2x80x9ciron-based powderxe2x80x9d encompasses powder essentially made up of pure iron; iron powder that has been pre-alloyed with other substances improving the strength, the hardening properties, the electromagnetic properties or other desirable properties of the end products; and particles of iron mixed with particles of such alloying elements (diffusion annealed mixture or purely mechanical mixture). Examples of alloying elements are copper, molybdenum, chromium, manganese, phosphorus, carbon in the form of graphite, and tungsten, which are used either separately or in combination, e.g. in the form of compounds (Fe3P and FeMo). Unexpectedly good results are obtained when the lubricants according to the invention are used in combination with iron-based powders having high compressibility. Generally, such powders have a low carbon content, preferably below 0.04% by weight. Such powders include e.g. Distaloy AE, Astaloy Mo and ASC 100.29, all of which are commercially available from Hoganas AB, Sweden.
Apart from the iron-based powder and the lubricant, the new powder composition may contain one or more additives such as binders, flow agents, processing aids and hard phases.
The binder may be added to the powder composition in accordance with the method described in U.S. Pat. No. 5,368,630 (which is hereby incorporated by reference) and may be organic compounds such as cellulose ester resins, hydroxyalkyl cellulose resins having 1-4 carbon atoms in the alkyl group, or thermoplastic phenolic resins.
A type of flow agent, which can be used according to the present invention, is disclosed in the U.S. Pat. No. 5,782,954 (which is hereby incorporated by reference). The flow agent, which is preferably a silicon dioxide, is used in an amount from about 0.005 to about 2 percent by weight, preferably from about 0.01 to about 1 percent by weight, and more preferably from about 0.025 to about 0.5 percent by weight, based on the total weight of the metallurgical composition. Furthermore, the flow agent should have an average particle size below about 40 nanometers. Preferred silicon oxides are the silicon dioxide materials, both hydrophilic and hydrophobic forms, commercially available as the Aerosil line of silicon dioxides, such as the Aerosil 200 and R812 products, from Degussa Corporation.
The processing aids used in the metal-powder composition may consist of talc, forsterite, manganese sulphide, sulphur, molybdenum disulphide, boron nitride, tellurium, selenium, barium difluoride and calcium difluoride, which are used either separately or in combination.
The hard phases used in the metal-powder composition may consist of carbides of tungsten, vanadium, titanium, niobium, chromium, molybdenum, tantalum and zirconium, nitrides of aluminium, titanium, vanadium, molybdenum and chromium, Al2O3, and various ceramic materials.
The invention is further illustrated by the following examples, which are to be interpreted only as examples but should not limit the scope of protection.